The present invention relates to a transfer system for transferring in a clean room a wafer cassette between semiconductor processing apparatuses for producing semiconductor devices such as very large-scale integrated circuits and large-scale integrated circuits.
In most of such conventional transfer systems, semiconductor processing apparatuses are placed within highly clean areas in a clean room, the areas being kept in a degree from 10 to 100 cleanliness class by using laminar air flow method. A wafer cassette which is loaded with wafers is transferred to a stacker crane of each highly clean area by means of an automatic wafer transport line using, for example, a linear motor carriage and then transported by a wafer automatic transfer robot to each of semiconductor processing apparatuses. This type of transfer system is disadvantageous in equipment cost and maintenance cost for producing a highly clean environment.
For overcoming these drawbacks, there has been, as illustrated in FIGS. 1 and 2, proposed a clean room using SMIF (TM) system, which includes a sealing cover 32 which covers over the top of a semiconductor processing apparatus 34, a clean air supply device 36 which incorporates both a high efficiency filter and a blower into it, a pair of SMIF arms (TM) 38 and 38 (hereinafter referred to as ARMs) which transfer wafer cassettes into and out of the semiconductor processing apparatus, and a pair of SMIF pods (TM) 40 (hereinafter referred to as pod). Each pod 40 includes a rectangular box door or removable bottom 42, on which a wafer cassette 44 is secured, and a box 46 which engages with the box door 42 for enclosing the wafter cassette 44 on the box door 42 in an airtight manner. Wafers 48 are loaded in the cassette 44 as illustrated in FIG 2. One pod 40 with wafer cassette 44 loaded with wafers 48 is manually placed on the top of one ARM 38 on the loading side and the other pod (not shown), having empty wafer cassette 44 received in it, is also manually placed on the top of the other ARM 39. Then, both the box door 42 and the wafter cassette 44, secured it, of each pod 40 are introduced into the corresponding ARM, from which only wafer cassette 44 is transferred into the semiconductor processing apparatus 34. Wafers 48 thus placed within the semiconductor processing apparatus 34 are sequentially subjected to processing and then transferred into the wafer cassette 44 on the unloading side. After this, the wafter cassette 44 loaded with the wafers 48 is returned back to the ARM 38 on the unloading side, where it is placed on the box door 42, which is then engaged with the box 42 placed on the top of that ARM 38 without exposing the wafers 48 to the atmosphere. The pod 40, containing the wafers 48 thus processed, is manually transported to another semiconductor processing apparatus for further processing. The following papers disclose the SMIF (TM) system: The Journal of Environmental Sciences, U.S.A., May/June '84 issue: "The Challenge to Control Contamination: A Novel Technique for the IC Process", page 23; Solid State Technology, U.S.A., July 1984, "SMIF, A Technology for Wafer Cassette Transfer in VLSI Manufacturing", page 111; and "Standard Mechanical Interface for Wafer Cassette Transfer" (Proposed), Document #1332, issued by Semiconductor Equipment and Materials Institute, Inc., U.S.A., Dec. 6, 1985.
The SMIF (TM) system ensures a high yield of product even in a clean room of a relatively low cleanliness, e.g., cleanliness class 1000 to 100000 since wafers are not allowed to be exposed to outside contamination and are placed in highly clean air during both processing and transportation thereof. However, both manual transportation of the pod 40 between semiconductor processing apparatuses and manual setting of pods 40 to respective ARMs 38, 39 are laborious.